Stencil-then-cover path rendering with shared edges

ABSTRACT

One embodiment of the present invention includes techniques for rasterizing primitives that include edges shared between paths. For each edge, a rasterizer unit selects and applies a sample rule from multiple sample rules. If the edge is shared, then the selected sample rule causes each group of coverage samples associated with a single color sample to be considered as either fully inside or fully outside the edge. Consequently, conflation artifacts caused when the number of coverage samples per pixel exceeds the number of color samples per pixel may be reduced. In prior-art techniques, reducing such conflation artifacts typically involves increasing the number of color samples per pixel to equal the number of coverage samples per pixel. Advantageously, the disclosed techniques enable rendering using algorithms that reduce the ratio of color to coverage samples, thereby decreasing memory consumption and memory bandwidth use, without causing conflation artifacts associated with shared edges.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of the U.S. patent applicationtitled, “STENCIL THEN COVER PATH RENDERING WITH SHARED EDGES,” filed onSep. 16, 2013 and having application Ser. No. 14/028,393, which claimspriority benefit of the U.S. Provisional Patent Application titled,“STENCIL-THEN-COVER PATH RENDERING WITH SHARED EDGES,” filed on Mar. 15,2013 and having Application No. 61/799,948. The subject matter of theserelated applications is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present invention relate generally to graphicsprocessing and, more specifically, to stencil-then-cover path renderingwith shared edges.

Description of the Related Art

Path rendering is a style of resolution-independent 2-dimensionalrendering, often called “vector graphics,” that is the basis for anumber of important rendering standards. Those standards includePostscript, Java 2D, Apple's Quartz 2D, OpenVG, PDF, TrueType fonts,OpenType fonts, PostScript fonts, HTML 5 Canvas, Scalable VectorGraphics (SVG) web format, Microsoft's Silverlight and Adobe Flash. forinteractive web experiences, Microsoft's XML Specification (XPS),drawings in Office file formats including PowerPoint, Adobe Illustratorillustrations, and more.

In path rendering, a software application typically specifies a scene asa sequence of resolution-independent outlines, known as paths, that maybe painted with constant colors, linear or radial gradients, or images.Many graphics processing units (GPUs) support rendering such pathsthrough a two-pass rendering process known as “stencil-then-cover.”First, in a path stenciling pass, the GPU generates a stencil bufferthat indicates which samples (i.e., positions within each pixel) arecovered by the path. Second, in a path covering pass, the GPU generatescover geometry for the path and shades the cover geometry with stenciltesting enabled. Typically, the GPU implements the cover geometry withsome sort of conservative bounding geometry, such as a bounding box(i.e., two triangles), a convex hull, or a similar construct. Becausestencil testing is enabled during the path covering pass, the GPUdiscards any fragments that are not covered by the path and writes thesurviving fragments to the color buffer as processed graphics data.

In some approaches to stencil-then-cover path rendering, the GPUmaintains N coverage samples per pixel and M color samples per pixel,where N is greater than M. In operation, the GPU determines the coverageof the path at N coverage samples per pixel. Then, during the pathcovering pass, before the GPU performs blending and color writeoperations, the GPU reduces the number of coverage samples to the numberof color samples. Notably, during this reduction operation, the GPU mayselect a group of N/M coverage samples associated with a single colorsample and generate an opacity value for the color sample based on theproportion of the N/M coverage samples that are covered. One limitationof this rendering approach is that reducing groups of multiple coveragesamples to single color samples may introduce conflation artifacts atedges that are shared between different paths. In particular, thereduction operation conflates the boolean concept of coverage with afractional value for opacity.

For example, suppose that there were 4 coverage samples corresponding toeach color sample (N=4 and M=1), the background color were blue, and theGPU were to be configured to render a yin-yang symbol—yin portion first.Further, suppose that the yin portion of the symbol were yellow, theyang portion of the symbol were green, and the yin and yang portionswere to share an edge at the boundary between the two portions. Finallysuppose that two of the coverage samples associated with a particularcolor sample were covered by the yellow yin portion of the symbol, andtwo of the coverage samples associated with the particular color samplewere covered by the green yang portion of the symbol. Upon rendering theyin portion of the symbol, the GPU would assign an intermediateblue-tinged color to the particular color sample—a blend of 50% of theyellow color associated with the yin portion of the symbol and 50% ofthe blue color associated with the background. Subsequently, uponrendering the yang portion of the symbol, the GPU would assign adifferent blue-tinged color to the particular color sample—a blend of50% of the green color associated with the yang portion of the symboland 50% of the intermediate blue-tinged color. Such background colorbleeding would degrade the quality of the displayed image. Inparticular, a fraction of the blue background color would be visible atthe boundary between the yin and yang portions of the yin-yang symbol(as if there were a gap between the yin and yang portions of theyin-yang symbol.)

In one approach to reducing the conflation artifacts associated withedges shared between different paths, the number of color samples isincreased to match the number of coverage samples. In suchimplementations, the coverage is maintained separately for each colorsample as a Boolean value and is not conflated with opacity.Consequently, rendering scenes that include edges that are sharedbetween different paths does not introduce conflation artifactsattributable to these shared edges. However, increasing the number ofcolor samples increases both the memory consumption and the memorybandwidth used by the color buffer. Because both the local memorycapacity of GPUs and the associated memory bandwidth is limited, anyincrease in either memory consumption or memory bandwidth use isgenerally undesirable.

Accordingly, what is needed in the art is a more effective approach torendering edges that are shared between different paths.

SUMMARY OF THE INVENTION

One embodiment of the present invention sets forth a method forrasterizing graphics primitives. The method includes determining arasterization state associated with a first edge of a graphicsprimitive, selecting a first sample rule from multiple sample rulesbased on the rasterization state, determining whether a first coveragesample is considered to be inside the first edge based on the firstsample rule, and determining whether the first coverage sample isconsidered to be covered by the graphics primitive based on whether thefirst coverage sample is considered to be inside the first edge.

One advantage of the disclosed approach is that a processing unit mayperform memory-efficient path rendering operations without introducingconflation artifacts that may be associated with edges that are sharedbetween different paths. Because the rasterizer unit considers eachgroup of coverage samples associated with a particular color sample tobe either fully inside or fully outside each shared edge, the number ofcoverage samples per pixel may exceed the number of color sampleswithout causing background bleeding in the displayed image.Consequently, the processing unit does not incur the increased memoryconsumption and memory bandwidth associated with prior-art techniques inwhich the number of color samples is increased to match the number ofcoverage samples.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram illustrating a computer system configured toimplement one or more aspects of the present invention;

FIG. 2 is a block diagram of a parallel processing unit included in theparallel processing subsystem of FIG. 1, according to one embodiment ofthe present invention;

FIG. 3 is a conceptual diagram of a graphics processing pipeline thatmay be implemented within the parallel processing unit of FIG. 2,according to one embodiment of the present invention;

FIG. 4 is a conceptual diagram illustrating a path stenciling pass and apath covering pass that the parallel processing unit of FIG. 2 may beconfigured to implement, according to one embodiment of the presentinvention;

FIG. 5 is a conceptual diagram illustrating a triangle fan that thegraphics processing pipeline of FIG. 3 may be configured to render,according to one embodiment of the present invention;

FIG. 6 is a conceptual diagram illustrating a sample group and sampleedge rules that the rasterizer unit of FIG. 3 may be configure toimplement, according to one embodiment of the present invention;

FIG. 7 is a conceptual diagram illustrating an index buffer, accordingto one embodiment of the present invention;

FIGS. 8A-8B set forth a flow diagram of method steps for rasterizingtriangles, according to one embodiment of the present invention; and

FIG. 9 is a flow diagram of method steps for rendering paths, accordingto one embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a more thorough understanding of the present invention. However,it will be apparent to one of skill in the art that the presentinvention may be practiced without one or more of these specificdetails.

System Overview

FIG. 1 is a block diagram illustrating a computer system 100 configuredto implement one or more aspects of the present invention. As shown,computer system 100 includes, without limitation, a central processingunit (CPU) 102 and a system memory 104 coupled to a parallel processingsubsystem 112 via a memory bridge 105 and a communication path 113.Memory bridge 105 is further coupled to an I/O (input/output) bridge 107via a communication path 106, and I/O bridge 107 is, in turn, coupled toa switch 116.

In operation, I/O bridge 107 is configured to receive user inputinformation from input devices 108, such as a keyboard or a mouse, andforward the input information to CPU 102 for processing viacommunication path 106 and memory bridge 105. Switch 116 is configuredto provide connections between I/O bridge 107 and other components ofthe computer system 100, such as a network adapter 118 and variousadd-in cards 120 and 121.

As also shown, I/O bridge 107 is coupled to a system disk 114 that maybe configured to store content and applications and data for use by CPU102 and parallel processing subsystem 112. As a general matter, systemdisk 114 provides non-volatile storage for applications and data and mayinclude fixed or removable hard disk drives, flash memory devices, andCD-ROM (compact disc read-only-memory), DVD-ROM (digital versatiledisc-ROM), Blu-ray, HD-DVD (high definition DVD), or other magnetic,optical, or solid state storage devices. Finally, although notexplicitly shown, other components, such as universal serial bus orother port connections, compact disc drives, digital versatile discdrives, film recording devices, and the like, may be connected to I/Obridge 107 as well.

In various embodiments, memory bridge 105 may be a Northbridge chip, andI/O bridge 107 may be a Southbrige chip. In addition, communicationpaths 106 and 113, as well as other communication paths within computersystem 100, may be implemented using any technically suitable protocols,including, without limitation, AGP (Accelerated Graphics Port),HyperTransport, or any other bus or point-to-point communicationprotocol known in the art.

In some embodiments, parallel processing subsystem 112 comprises agraphics subsystem that delivers pixels to a display device 110 that maybe any conventional cathode ray tube, liquid crystal display,light-emitting diode display, or the like. In such embodiments, theparallel processing subsystem 112 incorporates circuitry optimized forgraphics and video processing, including, for example, video outputcircuitry. As described in greater detail below in FIG. 2, suchcircuitry may be incorporated across one or more parallel processingunits (PPUs) included within parallel processing subsystem 112. In otherembodiments, the parallel processing subsystem 112 incorporatescircuitry optimized for general purpose and/or compute processing.Again, such circuitry may be incorporated across one or more PPUsincluded within parallel processing subsystem 112 that are configured toperform such general purpose and/or compute operations. In yet otherembodiments, the one or more PPUs included within parallel processingsubsystem 112 may be configured to perform graphics processing, generalpurpose processing, and compute processing operations. System memory 104includes at least one device driver 103 configured to manage theprocessing operations of the one or more PPUs within parallel processingsubsystem 112. The system memory 104 also includes a softwareapplication 125 that executes on the CPU 102 and may issue commands thatcontrol the operation of the PPUs.

In various embodiments, parallel processing subsystem 112 may beintegrated with one or more other the other elements of FIG. 1 to form asingle system. For example, parallel processing subsystem 112 may beintegrated with CPU 102 and other connection circuitry on a single chipto form a system on chip (SoC).

It will be appreciated that the system shown herein is illustrative andthat variations and modifications are possible. The connection topology,including the number and arrangement of bridges, the number of CPUs 102,and the number of parallel processing subsystems 112, may be modified asdesired. For example, in some embodiments, system memory 104 could beconnected to CPU 102 directly rather than through memory bridge 105, andother devices would communicate with system memory 104 via memory bridge105 and CPU 102. In other alternative topologies, parallel processingsubsystem 112 may be connected to I/O bridge 107 or directly to CPU 102,rather than to memory bridge 105. In still other embodiments, I/O bridge107 and memory bridge 105 may be integrated into a single chip insteadof existing as one or more discrete devices. Lastly, in certainembodiments, one or more components shown in FIG. 1 may not be present.For example, switch 116 could be eliminated, and network adapter 118 andadd-in cards 120, 121 would connect directly to I/O bridge 107.

FIG. 2 is a block diagram of a parallel processing unit (PPU) 202included in the parallel processing subsystem 112 of FIG. 1, accordingto one embodiment of the present invention. Although FIG. 2 depicts onePPU 202, as indicated above, parallel processing subsystem 112 mayinclude any number of PPUs 202. As shown, PPU 202 is coupled to a localparallel processing (PP) memory 204. PPU 202 and PP memory 204 may beimplemented using one or more integrated circuit devices, such asprogrammable processors, application specific integrated circuits(ASICs), or memory devices, or in any other technically feasiblefashion.

In some embodiments, PPU 202 comprises a graphics processing unit (GPU)that may be configured to implement a graphics processing pipeline toperform various operations related to generating pixel data based ongraphics data supplied by CPU 102 and/or system memory 104. Whenprocessing graphics data, PP memory 204 can be used as graphics memorythat stores one or more conventional frame buffers and, if needed, oneor more other render targets as well. Among other things, PP memory 204may be used to store and update pixel data and deliver final pixel dataor display frames to display device 110 for display. In someembodiments, PPU 202 also may be configured for general-purposeprocessing and compute operations.

In operation, CPU 102 is the master processor of computer system 100,controlling and coordinating operations of other system components. Inparticular, CPU 102 issues commands that control the operation of PPU202. In some embodiments, CPU 102 writes a stream of commands for PPU202 to a data structure (not explicitly shown in either FIG. 1 or FIG.2) that may be located in system memory 104, PP memory 204, or anotherstorage location accessible to both CPU 102 and PPU 202. A pointer tothe data structure is written to a pushbuffer to initiate processing ofthe stream of commands in the data structure. The PPU 202 reads commandstreams from the pushbuffer and then executes commands asynchronouslyrelative to the operation of CPU 102. In embodiments where multiplepushbuffers are generated, execution priorities may be specified foreach pushbuffer by an application program via device driver 103 tocontrol scheduling of the different pushbuffers.

As also shown, PPU 202 includes an I/O (input/output) unit 205 thatcommunicates with the rest of computer system 100 via the communicationpath 113 and memory bridge 105. I/O unit 205 generates packets (or othersignals) for transmission on communication path 113 and also receivesall incoming packets (or other signals) from communication path 113,directing the incoming packets to appropriate components of PPU 202. Forexample, commands related to processing tasks may be directed to a hostinterface 206, while commands related to memory operations (e.g.,reading from or writing to PP memory 204) may be directed to a crossbarunit 210. Host interface 206 reads each pushbuffer and transmits thecommand stream stored in the pushbuffer to a front end 212.

As mentioned above in conjunction with FIG. 1, the connection of PPU 202to the rest of computer system 100 may be varied. In some embodiments,parallel processing subsystem 112, which includes at least one PPU 202,is implemented as an add-in card that can be inserted into an expansionslot of computer system 100. In other embodiments, PPU 202 can beintegrated on a single chip with a bus bridge, such as memory bridge 105or I/O bridge 107. Again, in still other embodiments, some or all of theelements of PPU 202 may be included along with CPU 102 in a singleintegrated circuit or system of chip (SoC).

In operation, front end 212 transmits processing tasks received fromhost interface 206 to a work distribution unit (not shown) withintask/work unit 207. The work distribution unit receives pointers toprocessing tasks that are encoded as task metadata (TMD) and stored inmemory. The pointers to TMDs are included in a command stream that isstored as a pushbuffer and received by the front end unit 212 from thehost interface 206. Processing tasks that may be encoded as TMDs includeindices associated with the data to be processed as well as stateparameters and commands that define how the data is to be processed. Forexample, the state parameters and commands could define the program tobe executed on the data. The task/work unit 207 receives tasks from thefront end 212 and ensures that GPCs 208 are configured to a valid statebefore the processing task specified by each one of the TMDs isinitiated. A priority may be specified for each TMD that is used toschedule the execution of the processing task. Processing tasks also maybe received from the processing cluster array 230. Optionally, the TMDmay include a parameter that controls whether the TMD is added to thehead or the tail of a list of processing tasks (or to a list of pointersto the processing tasks), thereby providing another level of controlover execution priority.

PPU 202 advantageously implements a highly parallel processingarchitecture based on a processing cluster array 230 that includes a setof C general processing clusters (GPCs) 208, where C≥1. Each GPC 208 iscapable of executing a large number (e.g., hundreds or thousands) ofthreads concurrently, where each thread is an instance of a program. Invarious applications, different GPCs 208 may be allocated for processingdifferent types of programs or for performing different types ofcomputations. The allocation of GPCs 208 may vary depending on theworkload arising for each type of program or computation.

Memory interface 214 includes a set of D of partition units 215, whereD≥1. Each partition unit 215 is coupled to one or more dynamic randomaccess memories (DRAMs) 220 residing within PPM memory 204. In oneembodiment, the number of partition units 215 equals the number of DRAMs220, and each partition unit 215 is coupled to a different DRAM 220. Inother embodiments, the number of partition units 215 may be differentthan the number of DRAMs 220. Persons of ordinary skill in the art willappreciate that a DRAM 220 may be replaced with any other technicallysuitable storage device. In operation, various render targets, such astexture maps and frame buffers, may be stored across DRAMs 220, allowingpartition units 215 to write portions of each render target in parallelto efficiently use the available bandwidth of PP memory 204.

A given GPCs 208 may process data to be written to any of the DRAMs 220within PP memory 204. Crossbar unit 210 is configured to route theoutput of each GPC 208 to the input of any partition unit 215 or to anyother GPC 208 for further processing. GPCs 208 communicate with memoryinterface 214 via crossbar unit 210 to read from or write to variousDRAMs 220. In one embodiment, crossbar unit 210 has a connection to I/Ounit 205, in addition to a connection to PP memory 204 via memoryinterface 214, thereby enabling the processing cores within thedifferent GPCs 208 to communicate with system memory 104 or other memorynot local to PPU 202. In the embodiment of FIG. 2, crossbar unit 210 isdirectly connected with I/O unit 205. In various embodiments, crossbarunit 210 may use virtual channels to separate traffic streams betweenthe GPCs 208 and partition units 215.

Again, GPCs 208 can be programmed to execute processing tasks relatingto a wide variety of applications, including, without limitation, linearand nonlinear data transforms, filtering of video and/or audio data,modeling operations (e.g., applying laws of physics to determineposition, velocity and other attributes of objects), image renderingoperations (e.g., tessellation shader, vertex shader, geometry shader,and/or pixel/fragment shader programs), general compute operations, etc.In operation, PPU 202 is configured to transfer data from system memory104 and/or PP memory 204 to one or more on-chip memory units, processthe data, and write result data back to system memory 104 and/or PPmemory 204. The result data may then be accessed by other systemcomponents, including CPU 102, another PPU 202 within parallelprocessing subsystem 112, or another parallel processing subsystem 112within computer system 100.

As noted above, any number of PPUs 202 may be included in a parallelprocessing subsystem 112. For example, multiple PPUs 202 may be providedon a single add-in card, or multiple add-in cards may be connected tocommunication path 113, or one or more of PPUs 202 may be integratedinto a bridge chip. PPUs 202 in a multi-PPU system may be identical toor different from one another. For example, different PPUs 202 mighthave different numbers of processing cores and/or different amounts ofPP memory 204. In implementations where multiple PPUs 202 are present,those PPUs may be operated in parallel to process data at a higherthroughput than is possible with a single PPU 202. Systems incorporatingone or more PPUs 202 may be implemented in a variety of configurationsand form factors, including, without limitation, desktops, laptops,handheld personal computers or other handheld devices, servers,workstations, game consoles, embedded systems, and the like.

Graphics Pipeline Architecture

FIG. 3 is a conceptual diagram of a graphics processing pipeline 350that may be implemented within the PPU 202 of FIG. 2, according to oneembodiment of the present invention. As shown, the graphics processingpipeline 350 includes, without limitation, a primitive distributor (PD)355; a vertex attribute fetch unit (VAF) 360; a vertex, tessellation,geometry processing unit (VTG) 365; a viewport scale, cull, and clipunit (VPC) 370; a tiling unit 375, a setup unit (setup) 380, arasterizer (raster) unit 385; a fragment processing unit, alsoidentified as a pixel shading unit (PS) 390, and a raster operationsunit (ROP) 395.

The PD 355 collects vertex data associated with high-order surfaces,graphics primitives, and the like, from the front end 212 and transmitsthe vertex data to the VAF 360.

The VAF 360 retrieves vertex attributes associated with each of theincoming vertices from shared memory and stores the vertex data, alongwith the associated vertex attributes, into shared memory.

The VTG 365 is a programmable execution unit that is configured toexecute vertex shader programs, tessellation programs, and geometryprograms. These programs process the vertex data and vertex attributesreceived from the VAF 360 and produce graphics primitives for furtherprocessing within the graphics processing pipeline 350. Although notexplicitly shown, the VTG 365 may include, in some embodiments, one ormore of a vertex processing unit, a tessellation initializationprocessing unit, a task generation unit, a task distributor, a topologygeneration unit, a tessellation processing unit, and a geometryprocessing unit.

The vertex processing unit is a programmable execution unit that isconfigured to execute vertex shader programs, lighting and transformingvertex data as specified by the vertex shader programs. For example, thevertex processing unit may be programmed to transform the vertex datafrom an object-based coordinate representation (object space) to analternatively based coordinate system such as world space or normalizeddevice coordinates (NDC) space. The vertex processing unit may readvertex data and vertex attributes that is stored in shared memory by theVAF and may process the vertex data and vertex attributes. The vertexprocessing unit stores processed vertices in shared memory.

The tessellation initialization processing unit is a programmableexecution unit that is configured to execute tessellation initializationshader programs. The tessellation initialization processing unitprocesses vertices produced by the vertex processing unit and generatesgraphics primitives known as patches. The tessellation initializationprocessing unit also generates various patch attributes. Thetessellation initialization processing unit then stores the patch dataand patch attributes in shared memory. In some embodiments, thetessellation initialization shader program may be called a hull shaderor a tessellation control shader.

The task generation unit retrieves data and attributes for vertices andpatches from shared memory. The task generation unit generates tasks forprocessing the vertices and patches for processing by later stages inthe graphics processing pipeline 350.

The task distributor redistributes the tasks produced by the taskgeneration unit. The tasks produced by the various instances of thevertex shader program and the tessellation initialization program mayvary significantly between one graphics processing pipeline 350 andanother. The task distributor redistributes these tasks such that eachgraphics processing pipeline 350 has approximately the same workloadduring later pipeline stages.

The topology generation unit retrieves tasks distributed by the taskdistributor. The topology generation unit indexes the vertices,including vertices associated with patches, and computes (U,V)coordinates for tessellation vertices and the indices that connect thetessellated vertices to form graphics primitives. The topologygeneration unit then stores the indexed vertices in shared memory.

The tessellation processing unit is a programmable execution unit thatis configured to execute tessellation shader programs. The tessellationprocessing unit reads input data from and writes output data to sharedmemory. This output data in shared memory is passed to the next shaderstage, the geometry processing unit as input data. In some embodiments,the tessellation shader program may be called a domain shader or atessellation evaluation shader.

The geometry processing unit is a programmable execution unit that isconfigured to execute geometry shader programs, thereby transforminggraphics primitives. Vertices are grouped to construct graphicsprimitives for processing, where graphics primitives include triangles,line segments, points, and the like. For example, the geometryprocessing unit may be programmed to subdivide the graphics primitivesinto one or more new graphics primitives and calculate parameters, suchas plane equation coefficients, that are used to rasterize the newgraphics primitives.

The geometry processing unit transmits the parameters and verticesspecifying new graphics primitives to the VPC 370. The geometryprocessing unit may read data that is stored in shared memory for use inprocessing the geometry data. The VPC 370 performs clipping, culling,and viewport transform to determine which graphics primitives arepotentially viewable in the final rendered image and which graphicsprimitives are not potentially viewable. The VPC 370 then transmitsprocessed graphics primitives to the tiling unit 375.

The tiling unit 375 is a graphics primitive sorting engine that residesbetween a world space pipeline 352 and a screen space pipeline 354, asfurther described herein. Graphics primitives are processed in the worldspace pipeline 352 and then transmitted to the tiling unit 375. Thescreen space is divided into cache tiles, where each cache tile isassociated with a portion of the screen space. For each graphicsprimitive, the tiling unit 375 identifies the set of cache tiles thatintersect with the graphics primitive, a process referred to herein as“tiling.” After tiling a certain number of graphics primitives, thetiling unit 375 processes the graphics primitives on a cache tile basis,where graphics primitives associated with a particular cache tile aretransmitted to the setup unit 380. The tiling unit 375 transmitsgraphics primitives to the setup unit 380 one cache tile at a time.Graphics primitives that intersect with multiple cache tiles aretypically processed once in the world space pipeline 352, but are thentransmitted multiple times to the screen space pipeline 354.

Such a technique improves cache memory locality during processing in thescreen space pipeline 354, where multiple memory operations associatedwith a first cache tile access a region of the L2 caches, or any othertechnically feasible cache memory, that may stay resident during screenspace processing of the first cache tile. Once the graphics primitivesassociated with the first cache tile are processed by the screen spacepipeline 354, the portion of the L2 caches associated with the firstcache tile may be flushed and the tiling unit may transmit graphicsprimitives associated with a second cache tile. Multiple memoryoperations associated with a second cache tile may then access theregion of the L2 caches that may stay resident during screen spaceprocessing of the second cache tile. Accordingly, the overall memorytraffic to the L2 caches and to the render targets may be reduced. Insome embodiments, the world space computation is performed once for agiven graphics primitive irrespective of the number of cache tiles inscreen space that intersects with the graphics primitive.

The setup unit 380 receives vertex data from the VPC 370 via the tilingunit 375 and calculates parameters associated with the graphicsprimitives, including, without limitation, the color values, surfacenormal vectors, and transparency values at each vertex of the graphicsprimitive. The setup unit 380 then transmits processed graphicsprimitives to rasterizer unit 385.

The rasterizer unit 385 scan converts the new graphics primitives andtransmits fragments and coverage data to the pixel shading unit 390.Additionally, the rasterizer unit 385 may be configured to perform zculling and other z-based optimizations.

The pixel shading unit 390 is a programmable execution unit that isconfigured to execute fragment shader programs, transforming fragmentsreceived from the rasterizer unit 385, as specified by the fragmentshader programs. Fragment shader programs may shade fragments atpixel-level granularity, where such shader programs may be called pixelshader programs. Alternatively, fragment shader programs may shadefragments at sample-level granularity, where each pixel includesmultiple samples, and each sample represents a portion of a pixel.Alternatively, fragment shader programs may shade fragments at any othertechnically feasible granularity, depending on the programmed samplingrate.

In various embodiments, the fragment processing unit 460 may beprogrammed to perform operations such as perspective correction, texturemapping, shading, blending, and the like, to produce shaded fragmentsthat are transmitted to the ROP unit 395. The pixel shading unit 390 mayread data that is stored in shared memory.

The ROP unit 395 is a processing unit that performs raster operations,such as stencil, z test, blending, and the like, and transmits pixeldata as processed graphics data for storage in graphics memory via thememory interface 214, where graphics memory is typically structured asone or more render targets. The processed graphics data may be stored ingraphics memory, parallel processing memory 204, or system memory 104for display on display device 110 or for further processing by CPU 102or parallel processing subsystem 112. In some embodiments, the ROP unit395 is configured to compress z or color data that is written to memoryand decompress z or color data that is read from memory. In variousembodiments, the ROP unit 395 may be located in the memory interface214, in the GPCs 208, in the processing cluster array 230 outside of theGPCs, or in a separate unit (not shown) within the PPUs 202.

The graphics processing pipeline 350 may be implemented by any one ormore processing elements within PPU 202. For example, a streamingmultiprocessor (not shown) within one of the GPCs 208 of FIG. 2 could beconfigured to perform the functions of one or more of the VTG 365 andthe pixel shading unit 390. The functions of the PD 355, the VAF 360,the VPC 450, the tiling unit 375, the setup unit 380, the rasterizerunit 385, and the ROP unit 395 may also be performed by processingelements within a particular GPC 208 in conjunction with a correspondingpartition unit 215. Alternatively, graphics processing pipeline 350 maybe implemented using dedicated fixed-function processing elements forone or more of the functions listed above. In various embodiments, PPU202 may be configured to implement one or more graphics processingpipelines 350.

In some embodiments, the graphics processing pipeline 350 may be dividedinto a world space pipeline 352 and a screen space pipeline 354. Theworld space pipeline 352 processes graphics objects in 3D space, wherethe position of each graphics object is known relative to other graphicsobjects and relative to a 3D coordinate system. The screen spacepipeline 354 processes graphics objects that have been projected fromthe 3D coordinate system onto a 2D planar surface representing thesurface of the display device 110. For example, the world space pipeline352 could include pipeline stages in the graphics processing pipeline350 from the PD 355 through the VPC 370. The screen space pipeline 354could include pipeline stages in the graphics processing pipeline 350from the setup unit 380 through the ROP unit 395. The tiling unit 375would follow the last stage of the world space pipeline 352, namely, theVPC 370. The tiling unit 375 would precede the first stage of the screenspace pipeline 354, namely, the setup unit 380.

In some embodiments, the world space pipeline 352 may be further dividedinto an alpha phase pipeline and a beta phase pipeline. For example, thealpha phase pipeline could include pipeline stages in the graphicsprocessing pipeline 350 from the PD 355 through the task generationunit. The beta phase pipeline could include pipeline stages in thegraphics processing pipeline 350 from the topology generation unitthrough the VPC 370. The graphics processing pipeline 350 performs afirst set of operations during processing in the alpha phase pipelineand a second set of operations during processing in the beta phasepipeline. As used herein, a set of operations is defined as one or moreinstructions executed by a single thread, by a thread group, or bymultiple thread groups acting in unison.

In a system with multiple graphics processing pipeline 350, the vertexdata and vertex attributes associated with a set of graphics objects maybe divided so that each graphics processing pipeline 350 hasapproximately the same amount of workload through the alpha phase. Alphaphase processing may significantly expand the amount of vertex data andvertex attributes, such that the amount of vertex data and vertexattributes produced by the task generation unit is significantly largerthan the amount of vertex data and vertex attributes processed by the PD355 and VAF 360. Further, the task generation unit associated with onegraphics processing pipeline 350 may produce a significantly greaterquantity of vertex data and vertex attributes than the task generationunit associated with another graphics processing pipeline 350, even incases where the two graphics processing pipelines 350 process the samequantity of attributes at the beginning of the alpha phase pipeline. Insuch cases, the task distributor redistributes the attributes producedby the alpha phase pipeline such that each graphics processing pipeline350 has approximately the same workload at the beginning of the betaphase pipeline.

Please note, as used herein, references to shared memory may include anyone or more technically feasible memories, including, withoutlimitation, a local memory shared by one or more streamingmultiprocessors, or a memory accessible via the memory interface 214,such as a cache memory, parallel processing memory 204, or system memory104. Please also note, as used herein, references to cache memory mayinclude any one or more technically feasible memories, including,without limitation, an L1 cache, an L1.5 cache, and the L2 caches.

Path Rendering

The software application 125 of FIG. 1 may configure the graphicsprocessing pipeline 350 to perform various operations related togenerating pixel data based on graphics data supplied by CPU 102 and/orsystem memory 104. In particular, the software application 125 mayconfigure the graphics processing pipeline 350 to perform operationsrelated to path rendering. As outlined previously herein, path renderingis the basis for a number of important rendering standards, such as HTML5 Canvas and SVG. In operation, the PPU 202 implementsstencil-then-cover path rendering with N coverage samples per pixel andM color samples per pixel, where N is greater than M. In general, thesoftware application 125 defines a path and configures the PPU 202 toperform stencil-the-cover path rendering as two sequential renderingpasses—the path stenciling pass and the path covering pass.

FIG. 4 is a conceptual diagram illustrating a path stenciling pass 485and a path covering pass 495 that the PPU 202 of FIG. 2 may beconfigured to implement, according to one embodiment of the presentinvention. As previously disclosed herein, the PPU 202 is configured toperform stencil-then-cover path rendering. First, the PPU 202 performsthe path stenciling pass 485, thereby generating a stencil data fromstenciling pass 445. Subsequently, the PPU 202 initiates the pathcovering pass 495, thereby generating a rendered path 455.

In operation, the software application 125 of FIG. 1 specifies a pathand initiates the path stenciling pass 485. Upon receiving the pathspecification, the PPU 202 generates a path geometry 403 that representsthe path. As shown, the VPC unit 370, the rasterizer unit 385, the PSunit 390, and the ROP unit 395 process the path geometry 403, renderingthe stencil data from stenciling pass 445 to a stencil buffer. Inalternate embodiments, the PPU may process the path geometry in anytechnically feasible fashion using any available processing units in anycombination. The values in the stencil buffer indicate which coveragesamples are covered by the path. As previously disclosed herein, the PPU202 maintains N coverage sample per pixel and M color samples per pixel,where N is greater than M. During the path stenciling pass 485, thecoverage samples are updated, but the color samples are not updated.

After generating the stencil data from stenciling pass 445, the PPU 202generates a covering geometry 405 that conservatively covers the path.As shown, the VPC unit 370, the rasterizer unit 385, the PS unit 390,and the ROP unit 395 process the covering geometry 405, shading thecovered pixels and generating the rendered path 455. In alternateembodiments, the PPU may process the covering geometry in anytechnically feasible fashion using any available processing units in anycombination. During this process, the PPU 202 reduces the number ofcoverage samples to the number of color samples. In someimplementations, for each color sample, the PPU 202 identifies a groupof N/M coverage samples that are associated with the color sample andgenerates an opacity value for the color sample based on the proportionof the N/M coverage samples that are covered. For instance, if thenumber of coverage samples per pixel is 16 and the number of colorsamples per pixel is 4, then the PPU 202 associates groups of 4 coveragesamples with each color sample, and the possible opacity values are 0,0.25, 0.5, 0.75, and 1.0. Notably, the PPU 202 uses the stencil datafrom stenciling pass 455 included in the stencil buffer to discard anyfragments and coverage samples that are not covered by the path. The PPU202 then writes the surviving fragments to the color buffer as therendered path 455.

As previously disclosed herein, typical prior-art PPUs may determinethat a color sample is only partially covered by a graphics primitive ata shared edge. Consequently, such PPUs could introduce conflationartifacts at the shared edge. By contrast, the PPU 202 determines thateach color sample is either fully covered or uncovered by a graphicsprimitive at each shared edge. However, the PPU 202 may determine that aparticular color sample is only partially covered by a graphicsprimitive at an unshared edge. Consequently, the techniques disclosedherein enable the PPU 202 to effectively render paths using morecoverage samples than color samples, while ensuring that the renderedpaths do not include conflation artifacts associated with shared edges.

FIG. 5 is a conceptual diagram illustrating a triangle fan 500 that thegraphics processing pipeline 350 of FIG. 3 may be configured to render,according to one embodiment of the present invention. As previouslydisclosed herein, the PPU 202 is configured to render the path geometry403 and the covering geometry 405 as part of stencil-then-cover pathrendering. The triangle fan 500 may be included in either or both of thepath geometry 403 and the covering geometry 405.

As shown, the triangle fan 500 is a set of connected triangles thatshare a single vertex. As also shown, the triangle fan 500 includesshared edges 510, unshared edges 520, and internal edges 530. Inparticular, each of the shared edges 510 is also included in anotherpath (not shown.) For instance, each of the shared edges 510 may also beincluded in a different graphics primitive. By contrast, the unsharededges 520 and the internal edges 530 are not included in any otherpaths. The shared edges 510 may be identified in any technicallyfeasible fashion. For instance, the software application 125 of FIG. 1may explicitly identify the shared edges 510 as part of the pathspecification. In addition, the device driver 103 of FIG. 1 maydesignate shared edges 510 as part of processing the path specification.

As shown, the internal edges 530 are edges that are internal to thetriangle fan 500. Each of the internal edges 530 is shared betweenconnected triangles included in the triangle fan 500. The internal edges530 may be identified in any technically feasible fashion. For instance,the internal edges 530 may be identified based on the specification ofthe triangle fan 500.

In one embodiment, during the path stenciling pass 485, the PPU 202determines coverage relative to the shared edges 510 differently thancoverage relative to the unshared edges 520 and the internal edges 530.Subsequently, during the path covering pass 495, the PPU 202 determinescoverage relative to the internal edges 530 differently than coveragerelative to the shared edges 510 and the unshared edges 520. Inalternative embodiments, the PPU may be configured to identify anynumber of other edge characteristics and, subsequently, modify therendering process based on these edge characteristics. For instance, insome alternate embodiments, during the path covering pass, the PPU doesnot distinguish between shared edges, unshared edges, and internal edgeswhen determining coverage.

As persons skilled in the art will recognize, the triangle fan 500 is anexample of a particular graphics primitive that the graphics processingpipeline 350 may be configured to render. Other graphics primitives,such as triangle strips, may also include shared edges 510, unsharededges 520, and internal edges 530. Such graphics primitives areprocessed by the PPU 202 in a similar fashion.

FIG. 6 is a conceptual diagram illustrating a sample group 610 andsample edge rules that the rasterizer unit 385 of FIG. 3 may beconfigure to implement, according to one embodiment of the presentinvention. As previously disclosed herein, the PPU 202 supports Ncoverage samples per pixel and M color samples per pixel, where N isgreater than M. As shown, for each color sample, the PPU 202 identifiesthe sample group 610 of N/M coverage samples that are associated withthe color sample. Further, the PPU 202 designates one of the coveragesamples included in the sample group 610 as a primary sample 617, andthe remainder of the coverage samples included in the sample group 610as secondary samples 615.

As shown, FIG. 6 depicts the sample group 610 as a dotted square, theprimary sample 617 as a heavily outlined dot, and the secondary samples615 as lightly outlined dots. Further, FIG. 6 depicts covered samples asfilled and uncovered samples as unfilled. As also shown, each of thesample groups 610 includes a single primary sample 617 and threesecondary samples 615. The computer system 100 may determine theposition of the coverage samples and designate the primary coveragesample 617 in any technically feasible fashion. For instance, the devicedriver 103 may determine the positions of the coverage samples anddesignate the first coverage sample included in each sample group 610 asthe primary coverage sample 617. However, for each sample group 610, thecomputer system 100 selects the corresponding primary sample 617deterministically and consistently across paths. In alternateembodiments, the sample group may include any number, including zero, ofsecondary samples.

Advantageously, in one embodiment, the rasterizer unit 385 is configuredto select between different sample edge rules based on characteristicsassociated with each edge. Note that sample edge rules, also referred toherein as “sample rules,” are rasterization rules that may be applied toa coverage sample to determine whether the coverage sample is consideredto be inside an edge. For each edge, the rasterizer unit 385 applies theselected sample edge rule to the edge to determine the coverage of thecoverage samples included in the sample group 610. The sample edge rulesinclude, without limitation, an individual sample edge rule 455, aprimary sample edge rule 465, and an any sample edge rule 475. As inconventional rasterizing operations, the rasterizer unit 385 determinesthe coverage of the sample group 610 relative to a particular primitiveby determining the intersection or union of the coverage results relatedto each edge of the primitive.

In applying the individual sample edge rule 455, the rasterizer unit 385determines the coverage of each of the coverage samples included in thesample group 610 independently of the other coverage samples included inthe sample group 610. In particular, the rasterizer unit 385 determinesthe coverage of the primary sample 617 based on the position of theprimary sample 617 relative to the edge. Similarly, for each of thesecondary samples 615, the rasterizer unit 385 determines the coverageof the secondary sample 615 based on the position of the secondarysample 615 relative to the edge.

As shown, the rasterizer unit 385 applies the individual sample edgerule 455 to two sample groups 610 located on one edge of a triangle. Forthe left-hand sample group 610, the rasterizer unit 385 determines thatthe position of one of the secondary samples 615 is inside the edge ofthe triangle and is, consequently, covered. For the right-hand samplegroup 610, the rasterizer unit 385 determines that the positions of theprimary sample 617 and two of the three secondary samples 615 arelocated inside the edge of the triangle and are, consequently, covered.

In applying the primary sample edge rule 465, the rasterizer unit 385determines the coverage of all of the coverage samples included in thesample group 610 based on the position of the primary sample 617relative to the edge. Consequently, the rasterizer unit 385 determinesthat either all of the coverage samples included in the sample group 610are covered or all of the coverage samples included in the sample group610 are uncovered. Advantageously, determining coverage in this mannerensures that, for the color sample associated with the sample group 610,coverage is not conflated with opacity during the reduction from (N/M)coverage samples to a single color sample.

As shown, the rasterizer unit 385 applies the primary sample edge rule465 to two sample groups 610 located on one edge of a triangle. For theleft-hand sample group 610, the rasterizer unit 385 determines that theposition of the primary sample 617 is outside the edge of the triangleand, consequently, the primary sample 617 and all of the secondarysamples 615 are uncovered. For the right-hand sample group 610, therasterizer unit 385 determines that the position of the primary sample617 is inside the edge of the triangle and, consequently, the primarysample 617 and all of the secondary samples 615 are covered.

In applying the any sample edge rule 475, if the rasterizer unit 385determines that the position of the primary sample 617 or any of thesecondary samples 615 is inside the primitive relative to the edge, thenthe rasterizer unit 385 determines that all of the coverage samples arecovered. Determining coverage in this manner ensures that, for the colorsample associated with the sample group 610, coverage is not conflatedwith opacity during the reduction from (N/M) coverage samples to asingle color sample. Notably, the any sample edge rule 475 is moreconservative than the individual sample edge rule 455 and the primarysample edge rule 465.

As shown, the rasterizer unit 385 applies the any sample edge rule 465to two sample groups 610 located on one edge of a triangle. For theleft-hand sample group 610, the rasterizer unit 385 determines that theposition of one of the four coverage samples is inside the edge of thetriangle and, consequently, the primary sample 617 and all of thesecondary samples 615 are covered. For the right-hand sample group 610,the rasterizer unit 385 determines that the positions of three of thefour coverage samples are inside the edge of the triangle and,consequently, the primary sample 617 and all of the secondary samples615 are covered.

Again, the rasterizer unit 385 may be configured to apply differentsample edge rules to the edges of a primitive based on characteristicsof the edge. Further, the rasterizer unit 385 may be configured toselect different edge rules based on whether the PPU 202 is executingthe path stenciling pass 485 or the path covering pass 495. Although theexamples described in conjunction with FIG. 6 illustrate coveragerelative to a single edge, the rasterizer unit 385 determines thecoverage of the sample group 610 relative to a particular primitive bydetermining the intersection of the coverage results related to eachedge of the primitive.

In one embodiment, during the path stenciling pass 485, the rasterizerunit 385 applies the individual sample edge rule 455 to the unsharededges 520 and the internal edges 530. By contrast, the rasterizer unit385 applies the primary sample edge rule 465 to the shared edges 510.Since the rasterizer unit 385 determines that each sample group 610 iseither fully covered or fully uncovered relative to the shared edges510, conflation artifacts associated with shared edges in prior-arttechniques may be reduced.

During the path covering pass 495, the rasterizer unit 385 applies theany sample edge rule 475 to the unshared edges 520 and the shared edges510, and applies the primary sample edge rule 465 to the internal edges530. Again, because the rasterizer unit 385 determines that each samplegroup 610 is either fully covered or fully uncovered relative to theshared edges 510, conflation artifacts associated with shared edges inprior-art techniques may be reduced. Further, by applying the primarysample edge rule 465 to the internal edges 530, the rasterizer unit 385avoids double rasterization during the path covering pass 495.

In some embodiments of the present invention, the computer system 100may configure the PPU 202 to effectively apply the primary sample edgerule 465 on a per-path basis. In particular, if the computer system 100determines that any of the edges included in a path are shared edges510, then the computer system 100 configures the PPU 202 to determinethe coverage of the entire sample group 610 based on the position of theprimary sample 617.

In one embodiment, if the device driver 103 of FIG. 1 determines that apath includes shared edges 510, then the device driver 103 co-locatesthe positions of the coverage samples included in each of the samplegroups 610 when rasterizing the path. More specifically, before therasterizer unit 385 rasterizes a primitive associated with such a path,the device driver 103 reprograms the positions of all of the secondarysamples 615 to match the position of the associated primary sample 617.Consequently, for each sample group 610, the rasterizer unit 385associates a single position with all of the coverage samples. After therasterizer unit 385 rasterizes the primitive, the device driver 103restores the positions of the secondary samples 615 to the originallyassigned positions. As persons skilled in the art will recognize,operations involving co-locating and restoring the positions of thesecondary samples 615 may be reduced when the PPU 202 processes asequence of primitives that all include shared edges 510.

In another embodiment, if the computer system 100 determines that a pathincludes shared edges 510, the computer system 100 configures the PSunit 390 to assign a single output coverage to all of the samplesincluded in the sample group 510 via a sample mask. In operation, the PSunit 390 selects the output coverage of the primary sample 617, assignsthe selected coverage to each of the associated secondary samples 615,and writes the corresponding sample mask.

FIG. 7 is a conceptual diagram illustrating an index buffer 760,according to one embodiment of the present invention. As is well knownin the art, the index buffer 760 may be used to reduce the memory usedto store graphics primitives as part of indexed geometry rendering.

In operation, a graphics primitive may be represented by a sequence ofindex buffer entries 750 included in the index buffer 760. As shown,each index buffer entry 750 includes an edge flag 752 and a vertex index754. In general, each index buffer entry 750 includes N total bits ofwhich the most significant M bits store the edge flag 752 and theremaining (N-M) bits store the vertex buffer index 754. As inconventional indexed geometry rendering, the vertex index 754 is anindex into a vertex buffer (not shown) that includes per-vertex data.However, in prior-art approaches, typically all N bits included in eachindex buffer entry are allocated for storage of the vertex index.Advantageously, by repurposing the most significant bits of each indexbuffer entry 750 to represent the edge flag 752, each index buffer entry750 may store per-edge data in addition to the per-vertex data accessedvia the vertex index 754. By contrast, conventional techniques typicallyduplicate the vertex data for each edge.

For example, as shown, the triangle 705-0 may be defined in a clockwiseorder 720 by the three vertices 715-0, 715-1, and 715-2. As also shown,the triangle 705-1 may be defined in the clockwise order 720 by thethree vertices 715-3, 715-0, and 715-2. Further, the edge that connectsthe vertex 715-0 and the vertex 715-2 is shared between multiple paths.As shown, the index buffer 760 represents the two triangles 705-0 and705-1 using 8-bit index buffer entries 750, where the most significantbit stores the edge flag 752 and the remaining 7 bits store the vertexindex 754. In particular, an edge flag 752 value of ‘1’ indicates thatthe graphics primitive edge originating from the corresponding vertexindex 754 is one of the shared edges 510.

To further optimize memory use, the semantics of the edge flag 752 mayvary depending on the rendering pass. In the example index buffer 760,the PPU 202 is configured to implement the path stenciling pass 485, andan activated edge flag 752 indicates the origin vertex 715 of aparticular shared edge 510. By contrast, if the PPU 202 is configured toimplement the path covering pass 495, then an activated edge flag 752indicates the origin vertex 715 of a particular internal edge 530.

In alternate embodiments, any number of bits may be reserved for theedge flag, and the edge flag may represent any data associated withedges. In some embodiments, the rasterizer unit may be configured toselect between two sample edge rules based on the edge flag. In otherembodiments, the edge flag may represent several differentcharacteristics of the edge, such as shared versus unshared and internalversus external. As persons skilled in the art will understand,embodiments of the invention include any techniques that repurpose bitsincluded in the index buffer to guide the selection of sample edge rulesduring rasterizing operations.

FIGS. 8A-8B set forth a flow diagram of method steps for rasterizingtriangles, according to one embodiment of the present invention.Although the method steps are described with reference to the systems ofFIGS. 1-7, persons skilled in the art will understand that any systemconfigured to implement the method steps, in any order, falls within thescope of the present invention.

As shown, a method 800 begins at step 802, where the rasterizer unit 385receives an input triangle as part of the path stenciling pass 485. Atstep 804, the rasterizer unit 385 selects a particular sample group 610of coverage samples, and initializes an associated primary cover valueand associated secondary cover values to values of ‘1.’ The primarycover value and each of the secondary cover values represent whether theassociated coverage sample is considered to be covered by the inputtriangle. As disclosed previously herein, the PPU 202 maintains Ncoverage samples per pixel and M color samples per pixel, where N isgreater than M. Each sample group 610 is associated with a particularcolor sample. Further, one of the (N/M) coverage samples in the samplegroup 610 is the primary sample 617 and the remaining coverage samplesare the secondary samples 615. The computer system 100 may select theprimary sample 617 in any technically feasible fashion. However, foreach color sample, the computer system selects the corresponding primarysample 617 deterministically and consistently across paths. Notably, thenumber of secondary cover values is equal to the number of secondarysamples 615.

At step 806, the rasterizer unit 385 selects a first edge included inthe input triangle. At step 808, the rasterizer unit 385 evaluates theprimary sample 617 versus the selected edge. If the rasterizer unit 385determines that the position of the primary sample 617 is inside theinput triangle with respect to the selected edge, then the rasterizerunit 385 sets a primary edge cover value to ‘1.’ Conversely, if therasterizer unit 385 determines that the position of the primary sample617 is outside the input triangle with respect to the selected edge,then the rasterizer unit 385 sets the primary edge cover value to ‘0.’In general, the primary edge cover value represents whether the primarysample 617 is considered to be inside the selected edge of the inputtriangle. At step 810, if the rasterizer unit 385 determines that theselected edge is not one of the shared edges 510, then the method 800proceeds to step 812. The rasterizer unit 385 may determine whether theselected edge is one of the shared edges 510 in any technically feasiblefashion. For instance, in some embodiments, the rasterizer unit 385 mayperform a read operation on the edge flag 752 to determine whether theedge flag 752 is activated. At step 812, the rasterizer unit 385 appliesthe individual sample edge rule 455 to the selected edge. In operation,for each of the secondary samples 615, the rasterizer unit 385 sets thecorresponding secondary edge cover value based on whether the positionof the secondary sample 615 is inside the input triangle with respect tothe selected edge. In general, the secondary edge cover value representswhether the secondary sample 615 is considered to be inside the selectededge of the input triangle. The method 800 then proceeds to step 816.

If, at step 810, the rasterizer unit 385 determines that the selectededge is one of the shared edges 510, then the method 800 proceeds tostep 814. At step 814, the rasterizer unit 385 applies the primarysample edge rule 465 to the selected edge. In operation, for each of thesecondary samples 615, the rasterizer unit 385 sets the correspondingsecondary edge cover value to match the primary edge cover value.Advantageously, by determining that the sample group 610 is either fullycovered or fully uncovered by the input triangle, the rasterizer unit385 implements a Boolean concept of coverage for color samples that areassociated with shared edges 510.

At step 816, the rasterizer unit 385 performs a logical AND operationbetween the primary edge cover value and the primary cover value. Inthis manner, the rasterizing unit 385 composites the coverage of theselected edge with any previous per-edge coverage values. At step 818,for each secondary edge cover value, the rasterizer unit 385 performs alogical AND operation between the secondary edge cover value and thesecondary cover value. At step 820, if the rasterizer unit 385determines that the selected edge is not the third edge included in theinput triangle, then the rasterizer unit 385 proceeds to step 822.

At step 822, the rasterizing unit 285 selects the next edge included inthe input triangle, and the method 800 returns to step 808. Therasterizer unit 385 cycles through steps 808 through 822, selecting andapplying edge rules to each edge until the rasterizer unit 385 hasprocessed all three edges included in the input triangle. In alternateembodiments, the input triangle may be replaced by any input primitive,and the rasterizer unit 385 cycles through steps 808 through 822 untilthe rasterizer unit 385 has processed all of the edges included in theinput primitive. At step 820, if the selected edge is the third edgeincluded in the input triangle, then the primary cover value and thesecondary cover value represent the coverage of the sample group 610 bythe input triangle. In alternate embodiments, the rasterizer unit 385may process multiple sample groups or edges in parallel. In somealternate embodiments, the rasterizer unit 385 may select anyrasterization rule based on any technically feasible criterion, such asother flags included in the index buffer entry 750.

FIG. 9 is a flow diagram of method steps for rendering paths, accordingto one embodiment of the present invention. Although the method stepsare described with reference to the systems of FIGS. 1-7, personsskilled in the art will understand that any system configured toimplement the method steps, in any order, falls within the scope of thepresent invention.

As shown, a method 900 begins at step 902, where the softwareapplication 125 specifies a path. As part of defining the path, thesoftware application 125 designates any shared edges 510. At step 904,the device driver 103 determines whether the path includes any sharededges 510. The device driver 103 may identify shared edges 510 includedin the path in any technically feasible fashion. For instance, thedevice driver 103 may identify some shared edges 510 based on explicitcommands issued by the software application 125, and identify othershared edges 510 while generating path geometries.

At step 906, if the device driver 103 determines that the path includesone or more shared edges 510, then the method 900 proceeds to step 908.At step 908, for each color sample, the device driver 103 co-locates thecoverage samples included in the associated sample group 610. Morespecifically, for each sample group 610, the device driver 103 sets thepositions of all of the secondary samples 615 to match the position ofthe primary sample 617. In alternate embodiments, the device driver mayreprogram the position of the coverage samples in any technicallyfeasible fashion. At step 910, the rasterizer unit 385 processes thepath. Advantageously, because each sample group 610 is associated with asingle position, the rasterizer unit 285 determines that each colorsample is either fully covered or fully uncovered by each graphicsprimitive. At step 912, the device driver 103 restores the positions ofthe secondary samples 615 included in each sample group 610, and themethod 900 proceeds to step 916. At step 916, the PS unit 390 and theROP unit 395 complete the rendering pass, and the method 900 terminates.In alternate embodiments, the PS unit 390 and the ROP unit 395 performstep 916, completing the rendering pass, before the device driver 103performs step 912, restoring the positions of the secondary samples 615.

If, at step 906, the device driver 103 determines that the path does notinclude one or more shared edges 510, then the method 900 proceeds tostep 914. At step 914, the rasterizer unit 385 processes the path. Inparticular, the rasterizer unit 385 maintains N distinctly locatedcoverage sample per pixel and M color samples per pixel while processingthe path. The method 900 then proceeds to step 916. At step 916, the PSunit 390 and the ROP unit 395 complete the rendering pass, and themethod 900 terminates.

In sum, in one instance, the rasterizer unit applies different edgerules to the edges of a primitive based on characteristics associatedwith the path. In particular, in the path stenciling pass, therasterizer unit selects which edge rule to apply to each edge based onwhether the edge is shared between different paths. If the edge isshared between different paths, then the rasterizer unit applies aprimary sample edge rule to the edge. In applying the primary sampleedge rule to the edge, the rasterizer unit determines whether a group ofcoverage samples associated with a single color sample are collectivelyinside or outside the edge based on the position of a single coveragesample—the primary sample. By contrast, if the edge is not shared, thenthe rasterizer unit applies an independent sample edge rule to the edge.In applying the independent sample edge rule to the edge, the rasterizerunit determines whether each coverage sample is inside or outside theedge based on the position of the coverage sample. Subsequently, foreach coverage sample, if the rasterizer unit determines that thecoverage sample is inside each edge associated with the primitive, thenthe rasterizer unit determines that the coverage sample is covered bythe primitive.

Similarly, in the path covering pass, the rasterizer unit selects whichedge rule to apply to each edge of the primitive based on whether theedge is internal to the primitive. If the edge is internal to theprimitive, then the rasterizer unit applies the primary sample edge ruleto the edge. By contrast, if the edge is not internal to the primitive,then the rasterizer unit applies an any sample edge rule to the edge. Inapplying the any sample edge rule to the edge, if the rasterizer unitdetermines that the position of any coverage sample associated with acolor sample is inside the edge, then the rasterizer unit determinesthat all of the coverage samples associated with the color sample areinside the edge. Again, for each coverage sample, if the rasterizer unitdetermines that the coverage sample is inside all of the edgesassociated with the particular primitive, then the rasterizer unitdetermines that the coverage sample is covered by the primitive.

In a second instance, when rendering indexed geometry, the device driverrepurposes the most significant bits included in an index buffer entryto store edge flags. In general, a series of sequential index bufferentries included in an index buffer define a primitive. For instance, aseries of three sequential index buffer entries define a triangleprimitive. The device driver designates M bits of an N-bit index bufferentry to store edge flags, and (N−M) bits of the index buffer entry tostore a vertex index. As is well-known, the vertex index is an indexinto a vertex buffer containing vertex data. In path rendering, thesemantics of the edge flags varies depending on the rendering pass.During the path stenciling pass, an activated most-significant bit in aparticular index buffer entry indicates that the primitive edgeoriginating from the associated vertex index is shared between two ormore paths. During the path covering pass, an activated most-significantbit in a particular index buffer entry indicates that the primitive edgeoriginating from the associated vertex index is internal to theprimitive.

In a third instance, the device driver may reprogram the positions ofsecondary coverage samples based on characteristics associated with thepath. The device driver performs any such reprogramming before therasterizer unit processes paths as part of either the path stenciling orthe path covering passes. In particular, before rasterizing a particularpath, if the device driver determines that a particular path includesone or more edges that are shared between multiple paths, then thedevice driver co-locates the coverage sample positions associated witheach color sample. In operation, for each coverage sample group, thedevice driver reprograms each of the secondary sample positions to matchthe primary sample position. After the rasterizer unit performsoperations associated with the path stenciling or the path coveringpass, the device driver restores the secondary samples to their originalpositions.

One advantage of the above approaches is that conflation artifactsassociated with edges that are shared between different paths arereduced. In particular, for each shared edge, the rasterizer unit setseach group of coverage samples associated with a particular color sampleto be either fully covered or fully uncovered by each primitive.Consequently, rasterizing primitives that include shared edges does notproduce partial coverage that causes background bleeding when renderingusing more coverage samples per pixel than color samples per pixel.Further, rendering using more coverage samples than color samplesreduces memory consumption and memory bandwidth use compared toprior-art techniques in which the number of color samples is increasedto match the number of coverage samples. In one aspect of the aboveapproaches, memory consumption is further optimized by repurposing bitsincluded in the index buffer to store edge flags. Consequently, thedisclosed approaches enable memory-efficient path rendering operationswithout the shared edge conflation artifacts typically associated withprior-art techniques.

One embodiment of the invention may be implemented as a program productfor use with a computer system. The program(s) of the program productdefine functions of the embodiments (including the methods describedherein) and can be contained on a variety of computer-readable storagemedia. Illustrative computer-readable storage media include, but are notlimited to: (i) non-writable storage media (e.g., read-only memorydevices within a computer such as compact disc read only memory (CD-ROM)disks readable by a CD-ROM drive, flash memory, read only memory (ROM)chips or any type of solid-state non-volatile semiconductor memory) onwhich information is permanently stored; and (ii) writable storage media(e.g., floppy disks within a diskette drive or hard-disk drive or anytype of solid-state random-access semiconductor memory) on whichalterable information is stored.

The invention has been described above with reference to specificembodiments. Persons of ordinary skill in the art, however, willunderstand that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The foregoing description and drawingsare, accordingly, to be regarded in an illustrative rather than arestrictive sense.

Therefore, the scope of embodiments of the present invention is setforth in the claims that follow.

The invention claimed is:
 1. A graphics processing unit, comprising: atleast one memory; and a rasterizer that is coupled to the at least onememory and generates a path by: receiving data representing a graphicsprimitive, wherein the data identifies a plurality of vertexes anddefines one or more edges between the vertexes; identifying one or moresample groups associated with a first pixel, wherein each sample groupincludes a primary sample and one or more secondary samples; determiningthat an edge flag associated with a first edge of the one or more edgesindicates that the first edge is a shared edge; and in response,processing the first edge using a different sampling rule thannon-shared edges of the graphics primitive, wherein the differentsampling rule specifies which samples in a first sample group areconsidered to be covered by the graphics primitive based on whether theprimary sample of the first sample group is covered by the graphicsprimitive.
 2. The graphics processing unit of claim 1, wherein thedifferent sampling rule is applied by the rasterizer during a stencilingpass in a stencil-then-cover rendering process.
 3. The graphicsprocessing unit of claim 1, wherein all samples of the first samplegroup are considered to be covered by the graphics primitive if theprimary sample of the first sample group is inside the first edge. 4.The graphics processing unit of claim 3, further comprising processing afirst non-shared edge of the graphics primitive using another samplingrule that specifies that a given sample of the first sample group isconsidered to be covered by the graphics primitive if the given sampleis inside the first non-shared edge.
 5. The graphics processing unit ofclaim 4, further comprising determining a union of coverage resultsproduced by applying the different sampling rule to the first samplegroup and coverage results produced by applying the another samplingrule to the first sample group.
 6. The graphics processing unit of claim1, wherein the edge flag is included in a first entry of an index bufferused to access vertex information stored in a vertex buffer.
 7. Thegraphics processing unit of claim 6, wherein the edge flag comprises theM upper most bits of the first entry, where M is a positive integer. 8.The graphics processing unit of claim 1, wherein the pixel includes Ncoverage samples and M color samples, each of N and M being positiveintegers, and each sample group of the one or more sample groupsincludes N/M coverage samples that include the primary sample and theone or more secondary samples.
 9. A computer-implemented method forgenerating a path, the method comprising: receiving data representing agraphics primitive, wherein the data identifies a plurality of vertexesand defines one or more edges between the vertexes; identifying a firstsample group associated with a first pixel, wherein the first samplegroup includes a primary sample and one or more secondary samples;selecting a set of sampling rules to apply to the first sample groupbased on whether the graphics primitive is being processed in astenciling pass or a covering pass of a stencil-then-cover renderingprocess; applying the set of sampling rules to the first sample group togenerate coverage results indicating which samples of the first samplegroup are covered by the graphics primitive; and performing one or moreadditional rendering operations based on the coverage results.
 10. Thecomputer-implemented method of claim 9, wherein the graphics primitiveis being processed in a stenciling pass, a first edge of the of the oneor more edges comprises a shared edge, and the set of sampling rulesincludes a first sample rule that is applied to the first sample groupbased on the first edge and a second sampling rule that is applied tothe first sample group based on a second edge of the one or more edges.11. The computer-implemented method of claim 10, wherein, when applyingthe first sampling rule to the first sample group, every sample includedin the first sample group is considered to be covered by the graphicsprimitive if the primary sample is inside the first edge.
 12. Thecomputer-implemented method of claim 10, wherein, when applying thesecond sampling rule to the first sample group, a given sample includedin the first sample group is considered to be covered by the graphicsprimitive if the given sample is inside the first edge.
 13. Thecomputer-implemented method of claim 10, further comprising determiningthat an edge flag associated with first edge indicates that the firstedge is a shared edge.
 14. The computer-implemented method of claim 13,wherein the edge flag is included in a first entry of an index bufferused to access vertex information stored in a vertex buffer.
 15. Thecomputer-implemented method of claim 9, wherein the graphics primitiveis being processed in a covering pass, a first edge of the of the one ormore edges comprises a shared edge, a second edge of the one or moreedges comprises an unshared edge, and the set of sampling rules includesa first sample rule that is applied to the first sample group based onboth the first edge and a second edge.
 16. The computer-implementedmethod of claim 15, wherein, when applying the first sampling rule tothe first sample group, every sample included in the first sample groupis considered to be covered by the graphics primitive if any sampleincluded in the first sample group is inside the first edge.
 17. Thecomputer-implemented method of claim 15, wherein, when applying thefirst sample rule to the first sample group, every sample included inthe first sample group is considered to be covered by the graphicsprimitive if any sample included in the first sample group is inside thesecond edge.
 18. The computer-implemented method of claim 15, wherein athird edge of the one or more edges comprises an internal edge, the setof sampling rules includes as second sampling rule that is applied tothe first sample group based on the third edge, and, when applying thesecond sample rule to the first sample group, every sample included inthe first sample group is considered to be covered by the graphicsprimitive if the primary sample is inside the third edge.
 19. Thecomputer implemented method of claim 9, wherein the first pixel includesN coverage samples and M color samples, each of N and M being positiveintegers, and the first sample group includes N/M coverage samples thatinclude the primary sample and the one or more secondary samples.
 20. Asystem, comprising: at least one memory; and a graphics processing unitthat is coupled to the at least one memory and generates a path by:receiving data representing a graphics primitive, wherein the dataidentifies a plurality of vertexes and defines one or more edges betweenthe vertexes; identifying a first sample group associated with a firstpixel, wherein the first sample group includes a primary sample and oneor more secondary samples; selecting a set of sampling rules to apply tothe first sample group based on whether the graphics primitive is beingprocessed in a stenciling pass or a covering pass of astencil-then-cover rendering process; applying the set of sampling rulesto the first sample group to generate coverage results indicating whichsamples of the first sample group are covered by the graphics primitive;and performing one or more additional operations based on the coverageresults.